This invention was made with government support under Contract No. W-7405-ENG-36 awarded by the U.S. Department of Energy. The government has certain rights in the invention.
The present invention relates to composite structures for achieving high critical current densities in superconductive film tapes. Such composite structures can be a high critical current superconductive tape including a mixture of rare earth metal barium copper oxide compositions. Such composite structures can also be a multilayer structure for high critical current superconductive tapes where individual layers are formed in situ from a single mixture of rare earth metal barium copper oxide compositions.
Since their initial development, coated conductor research has focused on fabricating increasing lengths of the material, while increasing the overall critical current carrying capacity. Different research groups have developed several techniques of fabricating coated conductors. Regardless of which techniques are used for the coated conductors, the goal of obtaining highly textured superconducting thick films, such as YBa2Cu3O7xe2x88x92x (YBCO), with high supercurrent carrying capability on metal substrates remains. The use of thick superconducting films for coated conductors appears logical because both the total critical current and the engineering critical current density (defined as the ratio of total critical current and the cross-sectional area of the tape) are directly correlated with the thickness of the superconducting films.
It has been known for some time that the critical current density of a YBCO film is a function of film thickness for films on either single crystal wafers or polycrystalline nickelbased alloy substrates. A higher critical current density is achieved at a YBCO film thickness in the range of about 100 to about 400 nanometers (nm). On the other hand, critical current density tends to decrease with increasing YBCO film thickness. For example, Foltyn et al., Appl. Phys. Lett., 63, 1848-1850, 1993, demonstrated that critical current density saturates to a value of around 1 megaampere per square centimeter (MA/cm2) for YBCO films having a thickness beyond 2 micrometers (xcexcm) on single crystal substrates. Critical current density is lower for YBCO on polycrystalline metal substrates, mainly due to less superior in-plane texture of the YBCO films. The challenge is that adding more YBCO material beyond about 2 xcexcm using normal processing conditions on metal substrates does not contribute to the overall supercurrent carrying capability. This suggests that the supercurrent in such thick YBCO films on metal substrates is not uniformly distributed throughout the film thickness. It is now believed that a high defect density present in the top region of a thick YBCO film results in such a problem.
Despite the recent progress in production of superconductive tapes, continued improvements remain desirable in the magnitude of critical current properties.
It is an object of the present invention to provide superconducting tapes having high critical current values, i.e., greater than about 1 mega-ampere (M-amp).
It is another object of the present invention to provide superconducting tapes having high critical current values by use of a layer including a mixture of, e.g., YBCO and a second superconducting material such as EuBCO and the like, such mixtures having an optimal ratio for high critical current values.
It is yet another object of the present invention to provide multilayer superconducting tapes having alternating layers of, e.g., different rare earth metal barium copper oxide compositions where such a multilayer structure is formed in situ from a single mixture of at least two rare earth-barium-copper-oxide compositions.
To achieve the foregoing and other objects, and in accordance with the purposes of the present invention, as embodied and broadly described herein, the present invention provides a superconducting structure including a substrate and a film composed of a mixture of at least two superconducting rare-earth-barium-copper oxides thereon the substrate, said film from about 0.5 micron to at least about 4 microns in thickness and including a ratio of a first rare earth metal of yttrium to a second rare earth metal of from about 3 to 1 to from about 1.5 to 1, the film characterized as having a critical current for the film including the mixture of rare-earth metals greater than a critical current for any single rare-earth metal containing superconducting rare-earth-barium-copper oxide layer of about the same thickness alone.
The present invention further provides a multilayer superconducting structure including a substrate and alternating layers of a first rare-earth-barium-copper oxide and a second rare-earth-barium-copper oxide, said multilayer superconducting structure formed in-situ by deposition of a single mixture of said first rare-earth-barium-copper oxide and said second rare-earth-barium-copper oxide followed by annealing under a high partial pressure oxygen atmosphere.
The present invention further provides a superconducting structure including a substrate, a film of a superconducting rare-earth-barium-copper oxide upon the substrate, the film comprising a composite multilayer structure of a first layer of a mixture of superconducting rare-earth-barium-copper oxides from about 0.2 microns to about 2 microns in thickness, a layer of an insulating material selected from the group consisting of cerium oxide, strontium titanate, yttrium oxide, magnesium oxide, and yttria-stabilized zirconia, or of a conducting material such as lanthanum strontium cobalt oxide and strontium ruthenium oxide, and a second layer of a mixture of superconducting rare-earth-barium-copper oxides from about 0.2 microns to about 2 microns in thickness, the superconducting structure characterized as having a total combined thickness of superconducting rare-earth-barium-copper oxide layers of at least 2 microns and characterized as having a critical current for the composite multilayer structure greater than a critical current for a single mixed superconducting rare-earth-barium-copper oxide layer of about the same thickness, the single mixed superconducting rare-earth-barium-copper oxide layer including the same rare-earth metals as the composite multilayer structure.